Nanoporous metals have properties differing greatly from those of bulk metals, and show promise for a variety of noteworthy functions within the physical and chemical fields. For example, nanoporous metals exhibit large surface area and specific size effects, show promise of having superior electrical properties, physical and chemical properties, physical characteristics, and optical and electromagnetic effects, and are expected to be applied for use as catalysts and nanodevice nanostructures.
Supercapacitors (“SCs”), which combine the unique features of high power, high energy, and long life, have been the subject of attention as a halfway point between batteries and normal capacitors (see non-patent document 1: Winter, M.; Brodd, R. J., “What are batteries, fuel cells, and supercapacitors?” Chem. Rev. 104, 4245-4269 (2004); non-patent document 2: Simon, P.; Gogotsi, Y., “Materials for electrochemical capacitors” Nat. Mater. 7, 845-854 (2008); non-patent document 3: Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W., “Nanostructured materials for advanced energy conversion and storage devices” Nat. Mater. 4, 366-377 (2005); non-patent document 4: Kotz, R.; Carlen, M., “Principles and applications of electrochemical capacitors”, Electrochim. Acta 45, 2483-2498 (2000); non-patent document 5: Burke, A., “Ultracapacitors: Why, how, and where is the technology?”, J. Power Sources 91, 37-50 (2000); non-patent document 6: Miller, J. R.; Simon P., “Electrochemical capacitors for energy management”, Science 321, 651-652 (2008); non-patent document 7: Pech, D.; Brunet, M.; Durou, H.; Huang, P. H.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P., “Ultrahigh-power micrometer-sized supercapacitors based on onion-like carbon”, Nature Nanotech, 5, DOI: 10.1038/NNANO.2010.162 (2010)).
SCs have large specific capacitances as the result of two charge mechanisms, namely, double-layer capacitance (non-patent documents 2-5; non-patent document 8: Huang, J. S.; Sumpter, B. G.; Meunier, V., “Theoretical model for nanoporous carbon supercapacitors”, Angew. Chem. Int. Ed. 47, 520-524 (2008)) and pseudocapacitance performing a charge-transfer reaction (non-patent documents 2-5; non-patent document 9: Conway, B. E.; Birss, V.; Wojtowicz, J., “The role and utilization of pseudocapacitance for energy storage by supercapacitors”, J. Power Sources 66, 1-14 (1997); non-patent document 10: Rudge, A.; Davey, J.; Raistrick, I.; Gottesfeld, S.; Ferraris, J. P., “Conducting polymers as active materials in electrochemical capacitors”, J. Power Source, 47, 89-107 (1994)), these two phenomena occurring via a non-Faradaic process and a Faradaic process, respectively, at or near the electrode/electrolyte interface (non-patent documents 2-10). These two mechanisms are dependent on the active electrode material substance used in the SC, and may act separately or together (non-patent documents 2-5 and 9-10; non-patent document 11: Toupin, M.; Brousse, T.; Belanger, D., “Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor”, Chem. Mater. 16, 3184-3190 (2004); non-patent document 12: Pang, S. C.; Anderson, M. A.; Chapman, T. W., “Novel electrode materials for thin-film ultracapacitors: Comparison of electrochemical properties of sol-gel-derived and electrodeposited manganese dioxide”, J. Electrochem. Soc. 147, 444-450 (2000); non-patent document 13: Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L., “Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer”, Science 313, 1760-1763 (2006); non-patent document 14: Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G., “Printable thin film supercapacitors using single-walled carbon nanotubes”, Nano Lett. 9, 1872-1876 (2009); non-patent document 15: Pushparaj, V. L.; Shaijumon, M. M.; Kumar, A.; Murugesan, S.; Ci, L.; Vajtai, R.; Linhardt, R. J.; Nalamasu, O.; Ajayan, P. M., “Flexible energy storage devices based on nanocomposite paper”, Proc. Natl. Acad. Sci. USA 104, 13574-13577 (2007)).
Of the numerous electrode material substances currently available, pseudocapacitative transition metal oxides, typically manganese dioxide (MnO2), have been the object of intense scrutiny as one type of extremely promising electrode material substance due their high theoretical capacity, environmental friendliness, low costs, and natural abundance (non-patent document 11; non-patent document 16: Chang, J. K.; Tsai, W. T., “Material characterization and electrochemical performance of hydrous manganese oxide electrodes for use in electrochemical pseudocapacitors”, J. Electrochem. Soc. 150, A1333-A1338 (2003)).
Lithium-ion batteries (“LIBs”) are especially superior among energy storage media for their levels of power density per unit of volume or weight (non-patent document 25: J. M. Tarascon, M. Armand, Nature 2001, 414, 359; non-patent document 26: Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 1997, 276, 1395; non-patent document 27: J. Hassoun, S. Panero, P. Simon, P. L. Taberna, B. Scrosati, Adv. Mater. 2007, 19, 1632; non-patent document 28: K. T. Nam, D. W. Kim, P. J. Yoo, C. Y. Chiang, N. Meethong, P. T. Hammond, Y. M. Chiang, A. M. Belcher, Science 2006, 312, 885). In order to achieve a greater reversible capacity (although such larger capacity might only be available momentarily), efforts have been made to discover a substance based on metallic tin as an anode electrode in lieu of a carbon-based compound (non-patent document 29: Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652-5653; non-patent document 30: Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novak, P. Adv. Mater. 1998, 10, 725-763; non-patent document 31: Coward, G. R.; Leroux, F.; Power, W. P.; Ouvrard, G.; Dmowski, W.; Egami, T.; Nazar, L. F. Electrochem. Solid-State Lett. 1999, 2, 367-370; non-patent document 32: Crosnier, O.; Brousse, T.; Devaux, X.; Fragnaud, P.; Schleich, D. M. J. Power Sources 2001, 94, 169-174) due to the high electron conductivity (non-patent document 33: Nazri, G.-A.; Pistoia G., “Lithium Batteries Science and Technology”, Kluwer: Boston, 2004) and high theoretical capacity (990 mAh/g, equivalent to Li4.4Sn) of metallic tin. These values are as much as three times those of graphite carbon (372 mAh/g, equivalent to LiC6) (non-patent document 34: I. A. Courtney, J. R. Dahn, J. Electrochem. Soc. 1997, 144, 2045; non-patent document 35: M. Winter, J. O. Besenhard, Electrochim. Acta 1999, 45, 31).
However, the degradation in cycle properties that occurs when metallic tin is rendered into a shape suitable for use in LIBs is extremely problematic. This degradation arises primarily from pulverization, aggregation, and loss of electrical contact properties, leading to an effective change in volume (200% or greater) between charging and discharging (non-patent document 36: S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J. M. Tarascon, J. Electrochem. Soc. 2001, 148, A285; non-patent document 37: E. Shembel, R. Apostolova, V. Nagirny, I. Kirsanova, Ph. Grebenkin, P. Lytvyn, J. Solid St. Electrochem. 2005, 9, 96). The following three methods have primarily been offered as strategies for overcoming the problem of what is known as the pulverization of tin: reducing particle size; using a composite material substance; and selecting an optimized binder substance (non-patent document 26; non-patent document 38: N. Li, C. Martin, J. Electrochem. Soc. 2001, 148, A164; non-patent document 39: M. Wachtler, M. R. Wagner, M. Schmied, M. Winter, J. O. Besenhard, J. Electroanal. Chem. 2001, 12, 510; non-patent document 40: Y. Yu, L. Gu, C. Zhu, P. A. van Aken, J. Maier, J. Am. Chem. Soc., 2009, 131 15984; non-patent document 41: Y. Yu, L. Gu, C. Wang, A. Dhanabalan, P. A. van Aken, J. Maier, Angew. Chem. Int. Ed., 2009, 48, 6485). Two methods have been proposed to this end. The most common method for mitigating changes in volume or metal particle aggregation is to use an ultrapure metal-containing compound or active/inert composite alloy material substance (non-patent document 26; non-patent document 42: J. O. Besenhard, J. Yang, M. Winter, J. Power Sources 1997, 68, 87; non-patent document 43: J. Y. Lee, R. Zhang, Z. Liu, Electrochem. Solid-State Lett. 2000, 3, 167; non-patent document 44: J. Yang, M. Wachtler, M. Winter, J. O. Besenhard, Electrochem. Solid-State Lett. 1999, 2, 161). Another method is to construct a tin-based composite having a hollow structure, partially allowing for large changes in volume, and maintaining an electrical channel (non-patent document 29; non-patent document 45: H. G. Yang and H. C. Zeng, Angew. Chem., Int. Ed., 2004, 43, 5930; non-patent document 46: S. J. Han, B. C. Jang, T. Kim, S. M. Oh and T. Hyeon, Adv. Funct. Mater., 2005, 15, 1845). Electrodes having stable and high capacities of about 500 mAh/g have been reported very recently; these are manufactured by electrically depositing Sn—Ni on a nanoarchitectured copper substrate (non-patent document 47: J. Hassoun, S. Panero, P. Simon, P.-L. Taberna, B. Scrosati, Adv. Mater. 2007, 19, 1632).